1. Field of the Invention
The present invention relates to an electrode catalyst layer, a membrane electrode assembly and a fuel cell which include the electrode catalyst layer and a manufacturing method of the electrode catalyst layer. More specifically, the present invention relates to a manufacturing method of an electrode catalyst layer which uses a non-precious metal catalyst (or a non-platinum catalyst) and has a high power generation performance, such an electrode catalyst layer, a membrane electrode assembly and a fuel cell which include such an electrode catalyst layer.
2. Description of the Related Art
A fuel cell is a power generation system which produces electric power along with heat. A fuel gas including hydrogen and an oxidant gas including oxygen react together at electrodes containing catalyst in a fuel cell so that a reverse reaction of water electrolysis takes place. A fuel cell is attracting attention as a clean energy source of the future because of advantages such as high efficiency, a small impact on the environment and a low level of noise relative to conventional power generation systems. A fuel cell is classified into several types according to an ion conductor employed therein. A fuel cell which uses a proton-conductive polymer membrane is called a proton exchange membrane fuel cell (PEMFC), or a polymer electrolyte fuel cell (PEFC).
Among various fuel cells, a PEMFC (or PEFC), which can be used at around room temperature, is considered as a promising fuel cell for use in vehicles and household stationary power supply etc. and is being developed widely in recent years. In the PEMFC (or PEFC), a joint unit which has a pair of electrode catalyst layers on both sides of a polymer electrolyte membrane and is called a membrane electrode assembly (MEA) is arranged between a pair of separators, on each of which either a gas flow path for supplying a fuel gas including hydrogen to one of the electrodes or a gas flow path for supplying an oxidant gas including oxygen to the other electrode is formed. The electrode for supplying a fuel gas is called a fuel electrode or anode electrode whereas the electrode for supplying an oxidant gas is called an air electrode or cathode electrode. In general, each of these electrodes includes an electrode catalyst layer, in which a polymer electrolyte(s) and carbon particles on which a catalyst such as a noble metal of platinum group is loaded are stacked, and a gas diffusion layer which has gas permeability and electron conductivity.
Apart from other problems such as improving durability and output density etc., cost reduction is the most major problem for putting the PEMFC (or PEFC) into practical use.
Since the PEMFC (or PEFC) employs expensive platinum as the electrode catalyst at present, an alternate catalyst material is strongly desired to fully promote the PEMFC (or PEFC). As more platinum is used in the air electrode than in the fuel electrode, an alternative to platinum (namely, a non-precious metal catalyst) with a high level of catalytic performance for oxygen-reduction on the air electrode is, in particular, well under development.
A mixture of a noble metal and nitride of iron (a transition metal) described in Patent document 1 is an example of a non-precious metal catalyst for the air electrode (or oxygen electrode). In addition, a nitride of molybdenum (a transition metal) described in Patent document 2 is another example. These catalyst materials, however, have only an insufficient catalytic performance for oxygen-reduction and are dissolved in some cases in an acidic electrolyte.
On the other hand, Non-patent document 1 reports that a partially-oxidized tantalum carbonitride has both excellent stability and catalytic performance. It is true that this oxide type non-precious metal catalyst has a high level of catalytic performance for oxygen-reduction in itself but it remains necessary to find out an appropriate method to make it into the electrode catalyst layer because it is not loaded on carbon particles unlike platinum catalyst.
Moreover, Patent document 3 describes an MEA employing a non-precious metal catalyst. In Patent document 3, however, there is such a problem that a method to make the non-precious metal catalyst into an electrode catalyst layer is not suitable for non-precious metal catalyst since it is only a method which is described, for example, in Patent document 4 and Patent document 5 etc. and is conventionally used for platinum catalyst.    <Patent document 1>: JP-A-2005-044659.    <Patent document 2>: JP-A-2005-063677.    <Patent document 3>: JP-A-2008-270176.    <Patent document 4>: JP-A-H2-048632.    <Patent document 5>: JP-A-H5-036418.    <Non-patent document 1>: “Journal of The Electrochemical Society”, Vol. 155, No. 4, pp. B400-B406 (2008).